Friendly ping ...
On 2019/6/28 14:52, Zhiqiang Liu wrote:
> Friendly ping ...
>
> On 2019/6/25 11:13, Zhiqiang Liu wrote:
>> From: Zhiqiang liu <liuzhiqiang26@xxxxxxxxxx>
>>
>> In __do_softirq func, MAX_SOFTIRQ_TIME was set to 2ms via experimentation by
>> commit c10d73671 ("softirq: reduce latencies") in 2013, which was designed
>> to reduce latencies for various network workloads. The key reason is that the
>> maximum number of microseconds in one NAPI polling cycle in net_rx_action func
>> was set to 2 jiffies, so different HZ settting will lead to different latencies.
>>
>> However, commit 7acf8a1e8 ("Replace 2 jiffies with sysctl netdev_budget_usecs
>> to enable softirq tuning") adopts netdev_budget_usecs to tun maximum number of
>> microseconds in one NAPI polling cycle. So the latencies of net_rx_action can be
>> controlled by sysadmins to copy with hardware changes over time.
>>
>> Correspondingly, the MAX_SOFTIRQ_TIME should be able to be tunned by sysadmins,
>> who knows best about hardware performance, for excepted tradeoff between latence
>> and fairness. Here, we add sysctl variable max_softirq_time_msecs to replace
>> MAX_SOFTIRQ_TIME with 2ms default value.
>>
>> Note: max_softirq_time_msecs will be coverted to jiffies, and any budget
>> value will be rounded up to the next jiffies, which relates to CONFIG_HZ.
>> The time accuracy of jiffies will result in a certain difference
>> between the setting jiffies of max_softirq_time_msecs and the actual
>> value, which is in one jiffies range.
>>
>> Signed-off-by: Zhiqiang liu <liuzhiqiang26@xxxxxxxxxx>
>> ---
>> Documentation/sysctl/kernel.txt | 17 +++++++++++++++++
>> kernel/softirq.c | 8 +++++---
>> kernel/sysctl.c | 9 +++++++++
>> 3 files changed, 31 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
>> index f0c86fbb3b48..23b36393f150 100644
>> --- a/Documentation/sysctl/kernel.txt
>> +++ b/Documentation/sysctl/kernel.txt
>> @@ -44,6 +44,7 @@ show up in /proc/sys/kernel:
>> - kexec_load_disabled
>> - kptr_restrict
>> - l2cr [ PPC only ]
>> +- max_softirq_time_msecs
>> - modprobe ==> Documentation/debugging-modules.txt
>> - modules_disabled
>> - msg_next_id [ sysv ipc ]
>> @@ -445,6 +446,22 @@ This flag controls the L2 cache of G3 processor boards. If
>>
>> ==============================================================
>>
>> +max_softirq_time_msecs:
>> +
>> +Maximum number of milliseconds to break the loop of restarting softirq
>> +processing for at most MAX_SOFTIRQ_RESTART times in __do_softirq().
>> +max_softirq_time_msecs will be coverted to jiffies, and any budget
>> +value will be rounded up to the next jiffies, which relates to CONFIG_HZ.
>> +The time accuracy of jiffies will result in a certain difference
>> +between the setting jiffies of max_softirq_time_msecs and the actual
>> +value, which is in one jiffies range.
>> +
>> +max_softirq_time_msecs is a non-negative integer value, and setting
>> +negative value is meaningless and will return error.
>> +Default: 2
>> +
>> +==============================================================
>> +
>> modules_disabled:
>>
>> A toggle value indicating if modules are allowed to be loaded
>> diff --git a/kernel/softirq.c b/kernel/softirq.c
>> index a6b81c6b6bff..1e456db70093 100644
>> --- a/kernel/softirq.c
>> +++ b/kernel/softirq.c
>> @@ -199,7 +199,8 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
>>
>> /*
>> * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
>> - * but break the loop if need_resched() is set or after 2 ms.
>> + * but break the loop if need_resched() is set or after
>> + * max_softirq_time_msecs msecs.
>> * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
>> * certain cases, such as stop_machine(), jiffies may cease to
>> * increment and so we need the MAX_SOFTIRQ_RESTART limit as
>> @@ -210,7 +211,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
>> * we want to handle softirqs as soon as possible, but they
>> * should not be able to lock up the box.
>> */
>> -#define MAX_SOFTIRQ_TIME msecs_to_jiffies(2)
>> +unsigned int __read_mostly max_softirq_time_msecs = 2;
>> #define MAX_SOFTIRQ_RESTART 10
>>
>> #ifdef CONFIG_TRACE_IRQFLAGS
>> @@ -248,7 +249,8 @@ static inline void lockdep_softirq_end(bool in_hardirq) { }
>>
>> asmlinkage __visible void __softirq_entry __do_softirq(void)
>> {
>> - unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
>> + unsigned long end = jiffies +
>> + msecs_to_jiffies(max_softirq_time_msecs);
>> unsigned long old_flags = current->flags;
>> int max_restart = MAX_SOFTIRQ_RESTART;
>> struct softirq_action *h;
>> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
>> index 1beca96fb625..96ff292ce7f6 100644
>> --- a/kernel/sysctl.c
>> +++ b/kernel/sysctl.c
>> @@ -118,6 +118,7 @@ extern unsigned int sysctl_nr_open_min, sysctl_nr_open_max;
>> #ifndef CONFIG_MMU
>> extern int sysctl_nr_trim_pages;
>> #endif
>> +extern unsigned int max_softirq_time_msecs;
>>
>> /* Constants used for minimum and maximum */
>> #ifdef CONFIG_LOCKUP_DETECTOR
>> @@ -1276,6 +1277,14 @@ static struct ctl_table kern_table[] = {
>> .extra2 = &one,
>> },
>> #endif
>> + {
>> + .procname = "max_softirq_time_msecs",
>> + .data = &max_softirq_time_msecs,
>> + .maxlen = sizeof(unsigned int),
>> + .mode = 0644,
>> + .proc_handler = proc_dointvec_minmax,
>> + .extra1 = &zero,
>> + },
>> { }
>> };
>>
>
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> .
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